I got started on laying out the drive system. I had to take into account the crank arms contacting the front tire. Also had to limit the rearward movement of the cranks so the bar and chain will stay snug when at rest. I would also like to make a stop for the cranks going forward, so they can't touch the tire. With the height, I don't need to cut off the extra sprocket arms, I'm using the arm at 42" in the pic for the connection point.
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The alignment of the chain over the cog must be perfect or the chain will jump off. So I lined up the spring attachment, but that might have a different connection point after the board goes on. I dug out a 16 tooth cog in case I need a higher gear, 18 should be fine though.
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The piece of chain I'm using will be shorter, but enough for 6 inches of travel across the cog. The cranks, as they are now, will provide 3 1/2" of travel with each push. It would be over 4 inches if they went further forward, but in a turn would contact the front tire. If I need more travel in the chain, I can attach it higher up on the crank arm, but the hole is already there in this setup.
I plan to use a length of 1" flat bar, aluminum or steel, about 40" long to connect the cranks to the chain. It will run from the cranks to the chain, close to the frame and have bends at each end for alignment. It will probably be 2mm thick.
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It may need a guide to keep it in line, but the return spring may hold everything in line.